libdfu: pointer aliasing issues

[ybernat]

Compiling with clang, the following warnings appear for libdfu:

../thunderbolt_sw_2-fwupd/libdfu/dfu-cipher-xtea.c:102:17: warning: cast from 'guint8 *' (aka 'unsigned char *') to 'guint32 *' (aka 'unsigned int *') increases required alignment from 1 to 4 [-Wcast-align]
        guint32 *tmp = (guint32 *) data;
                       ^~~~~~~~~~~~~~~~
../thunderbolt_sw_2-fwupd/libdfu/dfu-cipher-xtea.c:144:17: warning: cast from 'guint8 *' (aka 'unsigned char *') to 'guint32 *' (aka 'unsigned int *') increases required alignment from 1 to 4 [-Wcast-align]
        guint32 *tmp = (guint32 *) data;
                       ^~~~~~~~~~~~~~~~

[hughsie]

Do you know how to fix this without copying the data buf to a new array? I guess the proper thing to do would be to allocate a guint32 array big enough to hold data and then to memcpy the data over, although that seems mighty inefficient.

[ybernat]

1. Is it expected that the data argument will be aligned in a way suitable for uint32? If so, you can just validate it by testing the address (but I'm not sure how to get the alignment of a type before C11 unless it's available as GNU extension in gcc).
2. Seems like may_alias attribute supported by gcc is relevant (I assume gcc generates a safe code in this case if needed, e.g. when compiling for ARM).
   https://gcc.gnu.org/onlinedocs/gcc-7.2.0/gcc/Common-Type-Attributes.html#Common-Type-Attributes